Vehicle transmissions are designed to transmit torque from an engine to a set of drive wheels in order to propel the vehicle within a range of output speeds. The engine includes an output shaft that may be selectively connected to a transmission input shaft when engine propulsion is required. In a manual transmission, a foot-operated clutch pedal may be actuated to allow a driver to shift gears and/or to place the transmission into a neutral state. In an automatic transmission, a hydrodynamic torque converter assembly or torque converter automatically provides the same engine/transmission connection.
A torque converter includes an impeller/pump, a turbine, and a stator. The torque converter is filled with oil. The pump, which may be bolted to a rotating flywheel portion of the engine to continuously rotate at engine speed, discharges the oil into the turbine. The turbine is connected to the transmission input shaft, so that any rotation of the turbine causes a rotation of the coupled transmission input shaft. A stator redirects oil discharged from the turbine back into the pump. The use of a torque converter thus enables a variable fluid coupling effect to occur automatically between the engine and the transmission, thereby allowing the vehicle to slow to a stop without stalling, while also allowing required torque multiplication to occur at low vehicle output speeds.
A torque converter may also include a lock-up torque converter clutch (TCC), a device configured to selectively lock the pump to the turbine above a calibrated threshold lockup speed. Below this lockup speed, an increasing amount of slip occurs across the torque converter as vehicle speed decreases, with a maximum slip level reached at zero vehicle speed. Regardless of whether a TCC is used, this variable slip capability allows the engine to continue to rotate when the vehicle is idling in certain transmission states or modes, e.g., in park (P), neutral (N), or in a drive state, i.e., a forward drive mode (D) or a reverse mode (R). In some transmission designs operating in a neutral (N) state during a drive detent position, i.e., when the vehicle reaches zero output speed at a standstill or when idling and the engine remains running, the transmission may be automatically shifted to a hydraulic neutral state referred to as neutral idle (NI).